Remanently magnetizable ferrite arrangement for providing directional attenuation of microwave transmission lines



Nov. 1, 1966 E. NECKENBURGER 3,283,268

REMANENTLY MAGNETIZABLE FERRITE ARRANGEMENT FOR PROVIDING DIRECTIONAL ATTENUATION 0F MICROWAVE TRANSMISSION LINES Filed July 18, 1963 REMANEN-TLY/ MAGNET/ZED FER/2 I TE INVENTOR. ERNST NECKENBURGER United States Patent 3,283,268 REMANENTLY MAGNETHZABLE FERRITE AR- RANGEMENT FER PROVIDING DIRECTIONAL ATTENUATTQN fill? MllCROWAVE TRANSMHS- SlON LENES Ernst Neckenhiirger, Wedel, Holstein, Germany, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed July 18, 1963, er. No. 295,962 Claims priority, application Germany, Aug. 9,

7 Claims. oi. ass-44.2

Microwave transmission line elements by which electromagnetic energy travelling in one direction is transmitted substantially without attenuation and electromagnetic energy travelling in the opposite direction is absorbed substantially without reflection, have important uses in microwave technology as directional lines or directional isolators, for example, to avoid load reactions on microwave generators. When the directional (or non-reciprocal) attenuation is effected by ferromagnetic resonance absorption, the bestknown embodiments comprise a wave-guide of rectangular cross-section and a strip of ferrite disposed therein at a suitable point. This strip is magnetized by a magnetic field at right angles to the direction of propagation of such strength that, in moperation with the microwave radiation in the wave-guide, ferromagnetic resonance is produced. The applied magnetic field, which at 10 mc./s. must have a value of about 3,000 oersteds, may be produced by a permanent magnet arranged outside the waveguide. At higher frequencies correspondingly higher fields are required, for example, of about 12,000 oersteds at 35x10 mc./s., which involves bulky arrangements. By the use of magnetic materials having a highly uni-axial crystal anistropy, such as the hard magnetic BaFe O the external magnetic field required for resonance may be considerably reduced or even dispensed with.

Another solution which may be used to avoid external magnetic fields is provided by magnetically soft materials having a substantially rectangular hysteresis loop and hence a high magnetic remanence. This remanence, however, attains its maximum value only in closed magnetic circuits. Embodiments of non-reciprocal transmis sion line elements using ferrites of this form and direction of magnetizaton are also known.

The use of highly remanent soft magnetic materials further enables the direction of magnetization to be reversed within a very short period of time with the aid of comparatively small field strengths, so that with nonreciprocal attenuation the forward direction and the reverse direction of the transmission line element are interchanged. This provides the possiblity of its use as a microwave switch or microwave modulator.

When in the known waveguide structures remanent and/or switchable non-reciprocal phase shifters or socalled field displacement isolators are used, the operating frequency of which lies far above the ferromagnetic resonance frequency, soft magnetic ferrites of cubic crystal structure and rectangular characteristics can be used to very high frequencies. If, however, ferromagnetic resonance losses are to be utilized to achieve directional attenuation, the operating frequency, which in 3,233,263 Patented Nov. 1, 1966 ice this case coincides with the ferromagnetic resonant frequency, is comparatively low in these materials.

Even when one bases oneself on the greatest values of saturation magnetization known for ferrites and assumes the remanent magnetization to be equal to the saturation magnetization, without external magnetic field resonant frequencies lower than 8x10 mc./ s. are obtained.

The present invention relates to a remanently magnetizable ferrite arrangement for providing directional attenuation of microwave transmission lines, which is characterized by the use of a thin-walled hollow cylinder which is adapted to be remanently magnetized in the circumferential direction and consists of polycrystalline ferrite material of hexagonal crystal structure, the individual crystallites of which have a preferred plane of magnetization and are oriented so that this preferred plane is at right angles to the axis of the hollow body extending in the direction of the microwave lines. With circumferential magnetization these hollow bodies have high remanence and a comparatively small coercive force but in the remanent state they are capable of showing a ferromagnetic natural frequency far exceeding that obtained by the use of cubic ferrite materials. The invention utilizes known properties and manufacturing methods of hexagonally crystallized ferrites having a preferred plane of magnetization. This mainly comprises the method of crystal orientation in which the preferred planes of the individual hexagonal crystallites are rendered substantially parallel.

Similarly to a single crystal the polycrystalline final product as a whole is magnetically anisotropic: in accordance with the degree of orientation it has a more or less marked preferred plane of magnetization. The remanent magnetization in this plane in the optimum case is 96% of the saturation magnetization. This is achieved on the base of the hexagonal crystal symmetry in the basal plane of the hexagonal crystallites, assuming the degree of orientation to be 100% and neglecting the stress anisotropy. With stress-free cubic crystallized ferrites the maximum re-manence to be expected is 87%.

The ferromagnetic natural resonance of a thinwalled hollow cylinder in accordance with the invention is when the height (i.e. axial length) of the body is large compared with its wall thickness, for example, when the body is tubular, and

when the height is equal to the wall thickness, for example, when the body is annular. H,(oe.) represents the crystal anisotropy field which, when the magnetization vector is deflected from the preferred plane, rotates it back into this plane, and M represents the remanent magnetization.

The first two columns of the Table 1 show values of H, and 41rM for a number of hexagonal ferrites having a preferred plane of magnetization.

The ferrites which are commercially available under the trade name Ferroxplana, for example C0 2, Co Y, Zn Y, Ni Y, Mg Y, where Z is an abbreviation for the formula of the radical Ba Fe O and Y for B21 Fe O are particularly suitable.

TABLE 1 H (0a.) 41rMS ftubc (Gauss) mc./

The third column of the Table 1 contains the natural frequencies calculated according to (la), where it is assumed that M, =0.96 of the saturation magnetization.

If H,, 2M, we have the approximate equality for the tube and the annulus respectively. When the ferrite lies in a magnetic field which varies in time and contains the components (11,, 12 h the elliptically polarized component of the alternating field which in the condition of ferromagnetic resonance is in maximum interaction with the precessing spin, is referred to as Larmor component. This component has the sense of rotation of the spin precession and along the circumference a certain ellipticity. The Larmor components are for the tube and the annulus respectively.

In a first approximation the anti-Larmor component has no interaction with the precessing spins. Its sense of rotation is opposite to the precession and its ellipticity is reciprocal to that of the Larmor component. The anti- Larmor components are.

(4a, b) I 41r1ll and a 27.1mm hZ 8+ rom for the tube and the annulus respectively.

In the case of ferromagnetic resonance an electromagnetic wave field guided through a transmission line and propagating in the direction of the axis of the ferrite cylinder is absorbed to a maximum extent when it has only a Larmor component at the cylinder wall; the absorption is a minimum if at this Wall there is only an anti-Larmor component of the Wave field. A reversal of the direction of propagation produces a change of the sense of rotation of the elliptical field component in the region of the ferrite. If for one direction of propagation the ferrite is located in a region of minimum absorption, a wave travelling in the opposite direction generally is not absorbed to a maximum extent; the regions of minimum forward attenuation and maximum reverse attenuation may be different.

However, from the theory of the resonance isolator in a Wave-guide of rectangular cross-over section it is known that the region in which the ratio between reverse attenuation and forward attenuation is a maximum substantially coincides with the region of minimum pass attenuation. In many cases this region can be calculated in known manner from the mathematical expressions for the undisturbed magnetic field components of the instantaneous wave-guide wave type by relating the values of the two magnetic field components at right angles to the direction of magnetization with the right-hand of the Equation 4a, b.

The drawing shows embodiments given by way of example.

FIGS. la and 1b each show a cross-sectional view of a circular wave-guide.

FIG. 2 is a longitudinal sectional view of a circular wave-guide,

FIG. 3 is a cross-sectional view of a rectangular waveguide.

A thin-walled hollow ferrite cylinder 1 of circular cross-section and having an inner radius p, which is disposed coaxially in a circular wave-guide 2 of radius R, produces an attenuation in this wave-guide which differs in accordance with the direction of propagation. The pass attenuation can be adjusted to a minimum and the ratio of blocking attenuation can be approximately adjusted to a maximum in accordance with p and R.

In FIGS. 1a and 1b m and m (dashed lines) denote the radii where a thin-walled ferrite tube made of the material Co Y may be located in order to achieve minimum forward attenuation. These regions are drawn approximately to scale in the cross-section of the circular Wave-guide 2, for the TE wave (FIG. la) and for the TE Wave (FIG. lb). When the TE wave is employed (FIG. 1b), the cylinder may be located in either of the regions m or m and it is also possible to locate a cylinder at each region if the cylinders at m and m are oppositely magnetized circumferentially. In this case the cylinder 1 is located at radius m and a cylinder 1 is located at the radius m The ferromagnetic resonance absorption of these arrangements occurs at about 22 10 mc./s.

A directional line for the HE Wave of a dielectric wave-guide is obtainable by concentrically surrounding the wave-guide for a certain distance by a metal tube. The diameter of this tube must be chosen so that it can only guide the fundamental wave of the screened dielectric wave-guide.

A radius of minimum pass attenuation and hence the radius of the ferrite tube in accordance with the invention can be calculated with the aid of the components of the fundamental wave of the screened dielectric line.

In the case of a helical directional wave-guide the Co Y tube must be arranged in accordance with the above conditions relating to the position of the tube within the turns of the wave-guide in order to obtain the maximum value of the ratio between the reverse attenuation and the forward attenuation.

In the design of the above described directional isolators which are rotationally symmetric about the axis of the waveguide the matching of the non-reciprocal waveguide parts is of particular importance. This matching may be effected with the aid of the usual means, for example, by smooth transitions in which, in addition to the ferrite, low-loss dielectric materials may be used. The directional line for the dielectric waveguide wave (HE type) may be surrounded by a metal tube the diameter of which progressively increases through a distance equal to several wavelengths to a value exceeding twice the limiting radius of the HE wave of the dielectric waveguide. A dielectric tube along the inner or outer periphery of the ferrite tube may serve to improve the quality of the directional waveguide. In order to avoid reflections, that is to say to provide matching, tapering dielectric branch tubes may be provided.

Matching of the directional lines for the TE wave and the HE wave of the circular wave guide may be effected in a similar manner. In the case of the TE wave the rotational symmetry has to be exactly maintained, since otherwise different wave types are generated, the occurrence of which detracts from the quality of the directional line.

The comparatively low coercive force in the preferred plane of hexagonally crystallized ferrites, for example of the type Ferroxplana, enables the directional magnetization to be reversed by the application of correspondingly small external fields. In this manner the forward direction and the reverse direction are interchanged for the above described directional waveguides. Thus the microwave energy can be switched within a very short time with only a small power. The circumferential magnetic field required for switching may be produced, for example, by a small number of wire turns wound on the cylinder in the axial direction. A current conductor which extends along the axis of the system reduces the inductance and hence the perturbation of the micro-wave field. Such an arrangement for switching a TE wave in the circular wave guide is shown schematically in FIG. 2 and comprises a waveguide 2, ferrite ll, matching elements 3 and a current conductor 4. The ferrite 1 is held within the guide by conventional means, such as by means of suitable supports (not shown) to the waveguide wall. If the current conductor is a very thin 'wire, the radius of minimum forward attenuation may be deduced in the above-described manner from the undisturbed magnetic field components of the TE wave. If the current load requires a thicker wire, under certain circumstances the magnetic field components of the T15 wave of the coaxial line have to be allowed for. The wire 4, which extends along the axis of the system, is bent at a sufficient distance from the ferrite towards the wall of the waveguide and is brought out in an electrically insulated manner through two small apertures 5 provided in this wall. The wire substantially does not reflect the TE wave if it lies in the plane extending at right angles to the electric field vector of the TE wave.

In the field of the TE wave of a rectangular waveguide it is impossible to arrange a ferrite body in accordance with the invention in a manner such that at the tube wall only the anti-Larmor component of the wave field occurs at all points, for in this case the loci of constant ellipticity of the magnetic field components are planes extending parallel to the smaller sides of the waveguide. Hence in a cross-section of the waveguide the loci for the anti-Larmor component are not closed lines but straight lines AA and BB extending parallel to the direction of magnetization, as shown in FIG. 3.

The straight lines together are loci of minimum pass attenuation only if ferrite strips 6a and 6b disposed in these regions are magnetized in opposite senses. A closed magnetic circuit may be produced through additional ferrite strips 7a and 7b. This magnetic circuit can be circumferentially magnetized to saturation by the central current conductor 4 and, when the current is switched off, shows a remanence corresponding to the material. Generally the strips 7a and 7b provide a greater contribution to the forward attenuation than the strips 6a and 6b, since in the region of the former strips the Larmorcomponent of the TE wave does not disappear. Accordingly the ratio between reverse attenuation and pass attenuation is slightly reduced.

The use of a ferrite tube of rectangular cross-section in accordance with the invention permits of using the described arrangement in the X band and at even higher frequencies as a resonance directional waveguide adapted to be switched. With a suificiently long thin-walled hollow tube the region of minimum forward attenuation can be exactly calculated for the wall parts 6a and 612. If

the tube is made of the material Co Y, the ferromagnetic resonance absorption lies at about 20 10 mc./s.

The quality of a resonance directional waveguide is determined by the maximum ratio between the reverse attenuation and the forward attenuation and also by the bandwidth of the absorption characteristic curve measured in the reverse direction between the 3 db points on either side of the ferromagnetic resonant frequency. Thus a corresponding quality factor is defined.

It has been found that in the rectangular waveguide with non-oriented ferrite with optimum shaping of the ferrite a quality factor of at most F =S.75 may be obtained. If in the ferrite arrangement in accordance with the invention as shown in FIG. 3 the effect of the wall sides 7a and 7b is neglected, with optimum location of the strips 6a and 6b quality factors between 10 and 30 are obtained wit-h the material values of Co Y. The use of a ferrite hollow cylinder in accordance with the invention consequently enables higher quality factors to be reached than can otherwise be obtained with non-oriented ferrites with otherwise identical geometry.

What is claimed is:

1. A non-reciprocal microwave device comprising a hollow longitudinally extending conductive waveguide for propagating microwave energy, a hollow body of a remanently magnetized polycrystalline ferrite material of hexagonal crystal structure positioned with said waveguide, the axis of said body extending parallel to the axis of said waveguide, the individual crystallites of said material having a preferred plane of magnetization and being oriented in said body with their preferred planes normal to the longitudinal axis of said body, said body being remanently magnetized circumferentially.

2. The device of claim 1 in which said waveguide is a circular waveguide and said body has a circular cross section and is coaxial with said waveguide.

33. The device of claim 2 in which the axial length of said body is large compared to its thickness, and said body has a radius such that the ratio of radial to axial components of magnetic field within said waveguide produced by said microwave energy is substantially equal to:

infil wherein H is the crystal anisotropy field which, when the magnetization vector is deflected from the preferred plane, rotates it back to said plane, and M is the remanent magnetization.

4. The device of claim 2 in which the axial length of said body is equal to the wall thickness of said body, and said body has a radius such that the ratio of radial to axial components of magnetic field produced by said microwave energy is substantially equal to:

wherein H, is the crystal anisotropy field which, when the magnetization vector is deflected from the preferred plane, rotates it back to said plane, and M is the remanent magnetization.

5. The device of claim 2 comprising a second hollow body of ferrite material of circular cross section within said waveguide, said second body being of the same mat'erial and crystal orientation as the first mentioned body, and being coaxial with and inside of said first mentioned body, said first mentioned and second bodies being circumferentially remanently magnetized in opposite directions.

6. The device of claim 1 in which said waveguide is a rectangular waveguide, and said body has a rectangular cross section with walls parallel to the adjacent walls of said waveguide.

7. The device of claim 1 comprising conductor means extending through said body for changing the remanent magnetization of said body.

References Cited by the Examiner UNITED OTHER REFERENCES Seidel: Anomalous Propagation in Ferrite-Loaded Waveguide, Proc. of the IRE, October 1956, p. 1411 relied on.

STATES PATENTS Truehaft et al.: Proc. of the IRE, August 1958, p. 1538.

58 Sooho: Theory and Application of Ferrites, Prenticeggfl n Hall, New Jersey, 1960, p. 217.

Anderson et a1. 33324.2 X

Zaleski 333*24'2 10 HERMAN KARL SAALBACH, Przmary Examiner. Schloemann 33324'2 P. L. GENSLER, Assistant Examiner.

Brockrnan et a1. 252-625 

1. A NON-RECIPROCAL MICROWAVE DEVICE COMPRISING A HOLLOW LONGITUDINALLY EXTENDING CONDUCTIVE WAVEGUIDE FOR PROPAGATING MICROWAVE ENERGY, A HOLLOW BODY OF A REMANENTLY MAGNETIZED POLYCRYSTALLINE FERRITE MATERIAL OF HEXAGONAL CRYSTAL STRUCTURE POSITIONED WITH SAID WAVEGUIDE, THE AXIS OF SAID BODY EXTENDING PARALLEL TO THE AXIS OF SAID WAVEGUIDE, THE INDIVIDUAL CRYSTALLITES OF SAID MATERIAL HAVING A PREFERRED PLANE OF MAGNETIZATION AND BEING ORIENTED IN SAID BODY WITH THEIR PREFERRED PLANES 